Zbynek_Janour
Institute of Thermomechanics AS CR, Dolejskova 5, Prague 8, 182 00, Czech republic
Summary
A street-canyon model having the aspect ratio B/H =1 and changeable
height H was designed and experimental methods are prepared for database
development. Laser sheet technique for quantitative analysis is prepared;
LDA method is modified for wind field measurements inside the environmental
wind tunnel and laser photoacustic spectroscopy method for concentrations
measurement is developed.
Aim of the research
Activities during the year
Approximate method for flow and dispersion in a street canyon was developed
and experimental methods was modified for wind tunnel experiments. Model
of the street-canyon with linen source was designed and tested.
Principal results
A special wind tunnel for the environmental flow simulations was built in the Institute of Thermomechanics (Balek and Janour, 1998) - see Fig. 1. It is open - circuit facility, which can operate as a fan driven
Fig. 1 Wind tunnel at the Institute of Thermomechanics
wind tunnel with unlimited running time now for velocities up to 13 m/s. It has the cross section 1,5 x 1,5 m2 and the length from the inlet 25,5 m. The tunnel inlet is formed by elliptic elements inside of a suction tower with 64 m2 screens and of size 6m x 6 m x 12 m. The long test section can naturally develop sufficiently thick shear layers in the working section. A 30kW blower drives the tunnel at the exit. Honeycombs, flow straighteners and screens placed at the entry guarantee that the wind tunnel will meet all demands on a aerodynamical facility of this type, e. g. the intensity of turbulence for the mean velocity more than 1 m/s was estimated below 0.4%.
The method of a approximate simulation was developed to simulate the flow and dispersion inside the street-canyon. According to it the roughness elements up stream of the street-canyon model simulates the surrounding town and Reynolds number independence method is used for the inside the street-canyon simulation.
A model of a street-canyon – see Fig. 2 was designed and constructed.
The model has aspect ratio B/H =1 and a changeable height H. The
Reynolds number Re=UH/n can be adjusted in the
interval (4 105; 5 106) for Reynolds number influence
investigation.
Fig. 2 Street-canyon model (A – line source)
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To solve and investigate all necessary flow and dispersion characteristics in aerodynamic wind tunnel, it was necessary to develop and to adapt some new methods of measurement. First of all, there was demand for flow visualisation and for application of such a method for rough introduction and approximation. Common system consisting from laser light sheet, Hi-Res. B/W CCD camera, suitable focusing system, frame grabber and Pentium based computer for processing is used. We were able to get really interesting results, with this, quite simple system. Of course, it was first necessary to optimise size and amount of particles used for visualisation, to find optimised parameters of laser light sheet, camera etc.
Next step in experiments is always quantitative analysis. It is known, that the measurement and modelling in environmental wind tunnel is quite complicated and it seems to be difficult to capture suitable data with standard instrumentation and process it formally. Care must be taken into account in utilisation of LDA system for measurement in environmental wind tunnel. Poor signal to noise ration and quality of signal under condition of turbulent flow, back reflection configuration, inhomogeneous seeding particles distribution etc. have influence during signal analysis and data processing. New BSA processors and a seeding particles generator (DANTEC) were bought to suppress these shortcomings.
For a thorough investigation of concentration variation one needs appropriate tools for near as well as far mean concentration field measurements. Therefore a high-resolution laser optoacoustic detection method was modified for concentrations wind tunnel measurements. Here CO2 laser and methanol as a trace quantity were applied. The method enabled a simple line source model was designed too. The source is hermetically closed vessel fitted with a membrane enabling the substance e. g. methanol contained to diffuse from the vessel more or less extensively. The substance in the vessel is in a liquid-vapour equilibrium, so that its vapour pressure inside the vessel remains constant over a long period of time, depending only on the substance and temperature. The bodies of the vessel – see Fig. 2, can be made of a polymer such as Teflon or polyethylene, serving as the membrane, and hermetically closed at the ends. The first experimental results for the case with the external velocity U = 1.5 m/s are demonstrated on Fig. 3
Fig. 3 Concentration measurements across the street-canyon
Main conclusions
The method of the simulation was put forward. The model of a street-canyon was designed and constructed, the high-resolution laser optoacoustic detection method was developed for concentration wind tunnel measurements and a simple line source model was designed.
Aim for the coming year
Adoption of the wind tunnel simulation to the urban boundary layer over streets inside the city with the line source.
Acknowledgements
The work was performed in co-operation with the Jaroslav Heyrovsky Institute of Physical Chemistry AS CR and was funded from the budget of the Grant Agency of the Academy of Sciences of the Czech Republic.
References
Balek V., Janour Z.; Atmospheric Boundary Layer Wind Tunnel
Modelling at the Institute of Thermomechnaics, 2nd
EECWE´98, Proc., Prague (1998), 201-206.